I think that's the punch he had landed earlier .....it looked exactly the same

I tried to verify but its hard to tell, it could be it because it was a right hook pac threw that JMM countered, its hard to tell though from the live angle.

Anyways while reviewing it, I took this screen a split second before the KO punch. Just looking at their positions and all of the leverage JMM got on that overhand right, it is not that surprising that Pac got KOed by this shot:

I mean really, getting KOed from the above shot doesnt mean Pac is shot, that his chin is gone or that JMM had to have been on peds. Just look at the punch, its a perfect counter with maximum leverage and pac coming forward into it.